Release of fascial compartment boundaries reduces muscle force output

Abstract

Most limb muscles operate within a compartment defined by fascial layers that enclose a muscle or groups of muscles within a defined space. These compartments are important clinically, because fluid accumulation can cause ischemia and tissue necrosis if untreated. Little is known, however, about how fascial enclosures influence healthy muscle function. One previous study showed that removing a fascial covering reduced the force output of a muscle under maximal stimulation. We hypothesized that such reduction in force output was due to a change in the muscle length following fasciotomy and that a reduced force output could be explained by the length-tension relationship of muscle. Thus we predicted that the maximum force across a range of lengths would be unchanged following fasciotomy. We measured maximal tetanic force output in a wing muscle in wild turkeys both before and after removal of fascia that enclosed the muscle in a compartment. Our hypothesis was not supported. The length-tension curve of this muscle showed that removal of fascia reduced maximum force output to 72 ± 10% of the prefascial release condition. Thus a reduction in muscle force following fasciotomy was not explained by a change in muscle length. The mechanism underlying reduction in force is unclear, but it suggests that the assumption underlying most isolated muscle experiments, i.e., removal of a muscle from its situation in vivo does not influence its maximal mechanical output, may need reexamining. NEW & NOTEWORTHY Most limb muscles are enclosed within compartments bound by robust fascial sheets. The mechanical significance of the close packing of muscle and fascia is largely unexplored. We used an animal model to show that removal of a fascial covering reduces the maximal force developed during contraction. These results raise questions about the use of isolated muscles to estimate muscle performance and suggest that a muscle's mechanical surrounding influences performance by mechanisms that are not understood.

Keywords: compartment; fasciotomy; intramuscular pressure; muscle; tendon.

Fig. 1.

Location of the region of interest within the wing (A) and ventral (B) and dorsal (C) views of turkey interosseous muscles. The muscles sit within a bony canal formed by the fused carpometacarpus and are encased by a fascial layer (not shown) on the dorsal and ventral surfaces. Muscle fibers insert on a long central tendon that attaches at the dorsal surface of the distal phalanx of digit III. Inset: photo shows the ventral interosseus with the cut edges of the fascia after fascial release.

Fig. 2.

Schematic of in situ preparation. The distal tendon of ventral interosseous was attached to a servomotor lever via Kevlar thread (red line). The wing was secured via clamps.

Fig. 3.

Sample contractions and length-tension curve. A: single contractions at optimal length show a decline in force from the intact condition (solid black line) to the postfasciotomy condition (gray line). The dotted line indicates the change in force in the intact condition from the first contraction to the last intact contraction. B: force produced across a range of lengths for the same individual (closed circles active; open circles passive), with the intact condition indicated by black and the postfasciotomy condition indicated by gray. The open diamond symbol is a repeat contraction, just before fasciotomy. The gray bar at A, bottom, indicates the time of stimulation.

Fig. 4.

Length-tension curves for intact and postfasciotomy conditions. Fascial release was associated with decreased muscle tension in active (closed circles) but not passive (open circles) conditions. Black symbols represent intact values, and gray represent postfasciotomy. Values are means ± SD for 5 animals.

Fig. 5.

Passive force-length curves for a single muscle during a ramp stretch before (black) and immediately after (gray) fasciotomy.